Method for fabricating shell molds for the production of superalloy castings

A method for the fabrication of shell molds suited for use in the production of directionally solidified components made from nickel and cobalt base alloys, comprising the steps of

• A. providing a pattern which duplicates the size and shape of the desired final product, said pattern being made of wax or similar material;
• B. providing a slurry containing
• from 15 to 25 parts of an aqueous solution containing about 50% aluminum polyoxychloride, from 6 to 10 parts water, from 8 to 14 parts of a binding agent such as latex, from 25 to 35 parts of -325 mesh alumina, from 7 to 12 parts of -100 mesh alumina, from 10 to 20 parts of -38/+100 mesh alumina, and from 0.5 to 1.5 parts of a pH control agent such as a 2% aqueous solution of HCl;
• C. dipping the pattern into the slurry so as to coat the pattern with the slurry;
• D. drying the coated pattern;
• E. dipping the coated pattern into the slurry and then applying dry -38/+100 mesh alumina;
• F. drying the coated pattern;
• G. dipping the coated pattern into ethyl alcohol, then into the slurry, and then applying dry -28/+48 mesh alumina;
• H. drying the coated pattern;
• I. repeating steps G and H a plurality of times;
• J. removing the wax by heating the coated pattern to a temperature in excess of the melting temperature of the pattern material;
• K. firing the mold at an elevated temperature to remove all pattern residue and consolidate the shell mold;
• whereby the resultant shell mold is composed of substantially pure alumina and is resistant to attack by molten nickel and cobalt alloys during exposures of up to 12 hours.